Nothing is syncing at all
Check the connection's status first. The commonest cause is an expired authorisation — some systems issue access that lasts an hour and must be refreshed, and a refresh can fail if the authorising account changed its password, lost a permission, or was deactivated.
A connection in that state shows as needing re-authorisation. Reconnecting fixes it and does not lose anything already synced.
Results are not being written back
Almost always a missing field on the record in your ATS. A write that names a field which does not exist is rejected entirely, which is why the score and the status disappear together.
Re-run the field check on the connection screen; it names what is missing.
Some candidates sync and others do not
Look at what the failures have in common rather than at the integration. Usually it is a required field empty on those particular records — no email address, no position reference — rather than anything intermittent.
Webhooks are not arriving
Check that your endpoint answers quickly with a 2xx. An endpoint that is slow or that returns an error is retried and then backed off. Your endpoint must also be reachable from the public internet; an address behind a VPN will never receive anything.
API calls started failing
A 401 means the key is wrong, revoked, or being sent against the internal /api/* routes
rather than /api/v1/*. A 429 means you are being rate limited and the response says for
how long. A 403 means the key is valid but lacks the scope for that call.
Still stuck
Open a support ticket with the connection name and roughly when it last worked. That window is what makes a log searchable, and it turns a day of guessing into a few minutes.